Decomposition of litter is a crucial process in terrestrial ecosystems, determining global carbon budget and nutrient turnover. Soil faunas have been shown to accelerate the rates of litter decomposition and nutrient cycling in terrestrial ecosystems. Litter decomposition has recently been observed in winter in alpine/subalpine ecosystems, but the contribution of soil fauna to the decomposition process is not clear. Field experiment using litterbags was conducted in order to quantify the contributions of soil fauna to mass losses of fir (Abies faxoniana) and birch (Betula albosinensis) litters during a freeze-thaw season in three representative alpine/subalpine forests. The litterbags of mesh sizes 0.02 mm, 0.125 mm, 1.0 mm and 3.0 mm were placed on the forest floor in October 2010, and collected after each of the three stages of the freeze-thaw season: OF, the onset of freezing stage (26 October to December 31); DF, the deeply frozen stage (1 January to 4 March 2011); and TS, the thawing stage (5 March to 30 April 2011) over the entire 2010/2011 winter. Over the whole freeze-thaw period, the mass losses of fir litter were 11–12% (0.02 mm), 12–13% (0.125 mm), 14–15% (1.0 mm) and 17–19% (3.0 mm), and that of birch litter were 10–13% (0.02 mm), 12–15% (0.125 mm), 13–18% (1.0 mm) and 17–22% (3.0 mm), respectively, depending on the altitude. The mass losses caused by microfauna, mesofauna and macrofauna for the fir litter accounted for 6–9, 12–13 and 22–25%, respectively and that for the birch litter accounted for 8–11, 13–15 and 25–27%, respectively. Furthermore, the contributions of soil fauna to mass loss showed an increasing trend with increasing body size regardless of species at three stages of the freeze-thaw period. These results suggest that soil fauna contributes strongly to litter decomposition during the freeze-thaw period in alpine/subalpine regions.